1. Field of the Invention
Apparatuses consistent with the present invention relate to a direct light type backlight unit and a liquid crystal display (LCD) apparatus employing the same and, more particularly, to a direct light type backlight unit capable of separating colors and a color filterless LCD apparatus employing the same.
2. Description of the Related Art
Typically, LCDs are used as flat panel display in notebooks, desktop computers, LCD-TVs, and mobile communication terminals. An LCD is a type of non-emissive flat panel display that needs external light to produce an image since it does not emit light. Thus, an LCD apparatus requires a backlight unit that is located behind a liquid crystal panel.
A liquid crystal panel simply transmits or blocks white light provided by a backlight unit. Thus, an LCD apparatus requires a color filter that transmits red R, green G, or blue B light in order to realize a color image. However, because each color element in the color filter transmits only light of a certain wavelength region, only about 30% of all light passing through the liquid crystal panel is transmitted. Taking into account the light loss due to other optical components, only less than about 10% of light irradiated by the backlight unit reaches a viewer. Because light loss caused by the color filter constitutes the largest percentage of the total light loss for the LCD apparatus, the color reproducibility of the LCD apparatus varies significantly depending on the performance of the color filter. Furthermore, since the color filter is expensive, the LCD apparatus has high manufacturing costs.
To overcoming these problems, a diffraction grating is used instead of a color filter to separate light into colored lights, thereby producing a color image.
Backlight units are classified into direct light type backlight units and edge light type backlight units according to the position of a light source arranged therein. In a direct light type backlight unit, a plurality of light sources disposed directly below an LCD panel emit light onto the LCD panel. In an edge light type backlight unit, a light source located along a sidewall of a light guide panel (LGP) emits light onto the LCD panel via the LGP.
U.S. Patent Application No. 2005-41174 discloses a conventional color filterless LCD apparatus for displaying a color image. The conventional color filterless LCD apparatus disclosed in the above-cited reference includes an edge light type backlight unit. Light emitted by a light source disposed along a side edge of the LGP and guided into the LGP at a predetermined angle is separated into R, G, and B color beams with different exit angles by a diffraction grating using the diffraction characteristics of the diffraction grating.
The R, G, and B color beams reach regions of a liquid crystal panel corresponding to color filter positions after passing through a cylindrical lens array. In this case, because the R, G, and B color beams are separately incident on the corresponding regions, it is possible to eliminate the use of a color filter that would otherwise be necessary for a typical liquid crystal panel.
The conventional color filterless LCD apparatus is restricted for use in compact display apparatus for the following reasons.
FIG. 1 shows separation of light according to its different wavelengths for a typical diffraction grating 1. Referring to FIG. 1, when light is incident on the diffraction grating 1 at a small incident angle α, angles βR, βG, and βB of R, G, and B color beams escaping from the diffraction grating 1 are large, thus making it difficult for the separate R, G, and B color beams to be incident on specific positions corresponding to the positions of a conventional color filter.
Thus, in order for the escaping R, G, and B color beams to be incident on precise required color positions within a liquid crystal panel, the incident angle α must be large. It is advantageous that G color beam has an exit angle of 0° and that R and B color beams have exit angles that are approximately symmetric about 0°. That is, the R, G, and B color beams may have exit angles such that they can be incident separately on the corresponding regions within the liquid crystal panel. For example, the R and B color beams may respectively have exit angles of −10° and +10° or −8° and +8°.
Thus, a color filterless display device known in the art uses an edge light type backlight unit with a LGP to obtain a significantly large incident angle of about 60° to 90°.
However, the edge light type backlight unit has problems that it is difficult for light to be incident on a diffraction grating at an optimum incident angle and to be uniformly delivered to a distal end of a LGP because the LGP guides light from one direction toward the other direction, thus resulting in poor light uniformity. Thus, a conventional filterless display device suffers a limitation to achieving a large-screen liquid crystal device.
In general, an edge light type backlight unit having a LGP can be used only in a compact LCD apparatus while a direct light type backlight unit is mainly used in a medium/large sized LCD apparatus.
Therefore, in order to realize a medium/large sized color filterless LCD apparatus using a direct light type backlight unit, it is necessary to make a beam incident on a diffraction angle at an angle suitable to achieve precise separation into component color beams.